Fuel cells are clean, efficient and an environmentally responsible power source for vehicles and various other applications. The fuel cell is under intense development as a potential alternative for the traditional internal-combustion engine used in modern vehicles. In proton exchange membrane (PEM) type fuel cells, a thin solid electrolytic membrane having an electrode with catalyst adjacent both sides forms a membrane electrolyte assembly (MEA). The MEA generally also includes porous conductive materials known as gas diffusion media (DM), which abut and distribute reactant gases to the anode and cathode. Hydrogen is supplied as fuel to the anode where it reacts electrochemically in the presence of catalyst to produce electrons and protons. The electrons are conducted by circuit from the anode to the cathode, and the protons migrate through the electrolyte to the cathode where oxygen reacts electrochemically in the presence of catalyst to produce oxygen anions. The oxygen anions react with the protons to form water as the fuel cell reaction product.
The MEA and DM, together with any insulating gaskets, make up a unitized electrode assembly (UEA), which is disposed between a pair of electrically conductive plates. The plates serve as current collectors for the electrodes and have appropriate openings, channels and passages formed therein for distributing the gaseous reactants over the respective electrodes, and for supplying coolant to the cell. PEM fuel cells are typically connected in series, stacked one on top of the other to form a fuel cell stack.
A fuel cell stack is ordinarily assembled under compression in order to seal the fuel cells and to secure and maintain a low interfacial electrical contact resistance between the reactant plates and the various components of the UEA. The interfacial contact resistances in a PEM fuel cell stack decrease substantially with increasing compression loading. A desired compression load on the fuel cell stack typically ranges from about 50 to about 400 psi, and is maintained by a compression retention enclosure housing the fuel cell stack. Compression retention systems are often built over-compressed to compensate for some loss in compression that occurs when the initial compression force is removed. In addition, the MEA is known to expand and contract with changes in humidity and temperatures; for example, in conventional fuel cell stacks, the MEA is known to expand by up to about 50% of its original thickness in operation. Compression retention enclosures must be designed to accommodate or cope with the strains produced by membrane swelling that can occur with both membrane expansion and compressive stress relaxation in the fuel cell stack.
A fuel cell stack assembly requires a significant amount of compressive force to squeeze the fuel cells of the stack together. The need for the compressive force comes about from the internal gas pressure of the reactants within the fuel cells plus the need to maintain good electrical contact between the internal components of the cells. Generally, the area per unit force is about 195-205 psi total, which is distributed evenly over the entire active area of the cell (typically 55-155 square inches for automotive size stacks). Thus, for a fuel cell with an area of about 80 square inches, the typical total compressive force of these size stacks is about 15,600 to 16,500 pounds.
Compression retention enclosures are designed to maintain a desired contact pressure between the bipolar plates, DM, and catalyst layers. A limited amount of compression of the DM is also known to occur under typical operational loads, however when excessive compression loads are applied to the DM, the force can physically degrade the DM by fracturing carbon fibers or breaking up binders that bind the carbon fibers together to an undesirable extent. Therefore, it is generally desirable for an appropriate compression load to be maintained and to provide a desired electrical resistance, but not to exceed the desired range during operation of the fuel cell stack. A compression retention enclosure typically includes a number of components coupled together and cooperating to maintain or retain compression on the fuel cell stack. Many different designs of compression retention enclosures exist, each offering one or more particular purported advantages over the other.
Conventional compression retention structural design focuses on the use of rigid end plates and tie rods to apply and maintain a compressive force on the fuel cell assembly. The plurality of fuel cells or fuel cell assembly to be compressed is interposed between a pair of rigid end plates. The end plates are then compressed together by tie rods that extend through or around the end plates and impart a compressive force on the end plates. Additionally, the tie rods typically extend beyond the surface of the end plates and thereby increase the volume of the stack structure. When the stack structure utilizes tie rods distributed around a periphery of the end plate to impart a compressive force on the fuel cell assembly, the proper tightening of the tie rods to impart the desired compressive force can be difficult. That is, the tie rods must be tightened in a predetermined pattern in order to attempt to apply in an evenly distributed compressive load on the fuel cell assembly. However, as each tie rod is tightened the compressive load being imparted by the end plates changes so that each tie rod must be re-tightened multiple times in an iterative process in order to achieve a generally uniform compressive force on the fuel cell assembly. Additionally, the tie rods typically extend beyond the surface of the end plates and thereby increase the volume of the stack structure.
Conventional fuel cell stack enclosures used bolted connections to retain stack load. The bolts thread into aluminum castings inside end unit assemblies, adding significant weight and bulk to the system. Further, since assembled length varies from part to part, the location of the wet end unit, which is the platform in which the lower end unit, reactant manifolds and balance of parts build from, changes, and drives design complexities in the balance of the plant. Slip joints may be required to tolerate stack height variation. Aside from making the structure difficult to environmentally seal, the use of large bolted joints adds a failure mode associated with the joints. Increased assembly time and costs, as well as increased packaging space are pragmatic considerations with conventional designs.
It would be advantageous to provide a stack structure that can more easily impart a compressive force on the fuel cell assembly, and even more advantageous if the compressive force applying means added minimal volume to the stack structure. Furthermore, it would be advantageous to provide a compression retention enclosure that retains sufficient compressive force while environmentally sealing the fuel cell assembly, and which also adds a minimal volume. It would also be advantageous to provide a fuel cell assembly with a compression retention enclosure effective in accommodating strains produced by operational membrane swelling and compressive stress relaxation in the fuel cell stack.
It would be desirable to develop a simplified design for a compression retention enclosure which minimizes the number of components required to maintain compression of the fuel cell stack, and which minimizes the mass of the fuel cell system, without compromising desired tolerances.